CLASSIFICATION OF THE PRIMORDIA 



73 



In this example it is useless to complicate the question by 

 numbering the cells ; this becomes necessary in certain more 

 compHcated cases. 



Since the two kinds of cells (units x + i) have the same'origin 

 (the egg), they are identical with regard to their hereditary 



r^ 



possibilities (§ 29). The observed differences 

 ought therefore to be ascribed to plasticity — 

 that is to say, to causes external with regard 

 to each cell. We may look upon the individual 

 X as being a society of individuals x-{-i (cells). 

 Each member of the society undergoes the 

 influence of two sorts of external causes : 

 (i) EXTERNAL causes properly so called, 

 such as temperature, light, etc. ; (2) SOCIAL 

 causes which emanate from other members of 

 the society or from the society taken as a 

 whole. In other words, each member (cell) 

 lives in a social environment: the differences 

 brought about by the influence of this en- 

 vironment are SOCIAL differences (see below, 

 Remark). 



According to this, it is obvious that the 

 difference between the intermediate and the 

 terminal cells of Spirogyra is a social difference. 

 It may be remarked that the longitudinal line 

 (axis) of segmentation is also a line of differentia- 

 tion. Indeed, following this line from one end to _ 

 another, we observe the variation produced hy -piG. 2.— Uniaxial 

 differentiation. The Hne of segmentation and system {Spirogyra 

 differentiation is already found in the eerg : Zygnema,etc.),di3.- 



it is the longitudinal axis. grammatic 



The principles, the application of which one finds in 

 Spirogyra in a very simple form, govern the distribution of the 

 properties in all organisms, from the simplest to the most 

 complicated. 



REMARK : The terms social cause, social efivironment and 

 social difference may render good service in biology. 



A very simple example of social differentiation is found in 

 the pseudo-bulb of Pha]us grandiflorus. The members (cells) of 

 this society (individual x) which are near the surface are green 

 because their so-called amyloplasts are impregnated with 

 chlorophyll. At a certain distance from the surface the 

 amyloplasts are colourless and the cells are white, because they 

 are deprived of the influence of light by the presence of the 

 external cells. Here the combined (simultaneous) action of an 

 external cause (light) and a social cause is obvious : the observed 

 difference is a social difference. 



O' 



